The present invention relates to telecommunications system and, in particular, to wireless communications system using a common transmission line to connect a plurality of base stations to a base station controller.
Wireless communication systems generally use transmission lines that are configured in a channelized format, such as T1 or E1 facilities, to connect a set of base stations to a base station controller (BSC). FIG. 1 illustrates a wireless communication system 10 in which a BSC 12 is individually connected to base stations 14-i via respective T1 facilities 16-i, where i=1, 2, . . . , n. Each of the T1 facilities 16-i includes a downlink path 17-i and an uplink path 19-i, as shown in FIG. 2, for the transmission of data to and from the base stations 14-i, respectively. Each of the paths 17-i, 19-i is configured into a channelized formatxe2x80x94that is, the paths are divided into transmission time slots. FIG. 3 illustrates a manner for channelizing the paths 17-i, 19-i. Each of the paths 17-i, 19-i is divided into frames having a framing time slot and twenty-four DS0j time slots, where j=1, 2, . . . , 24 and each of the DS0j time slots has a 64 Kbps bandwidth. Thus, the wireless communication system 10 has a 24xc2x7n number of DS0j time slots in the downlink direction and 24xc2x7n number of DS0j time slots in the uplink direction.
The DS0j time slots are used to form communication channels, including a signaling channel and a plurality of traffic channels. Each communication channel includes at least one DS0j time slot in the downlink path 17-i and the uplink path 19-i. For purposes of discussion, a DS0j time slot shall be hereinafter deemed to refer to both a DS0j time slot in the downlink path and a DS0j time slot in the uplink path, unless otherwise specified.
Each of the base stations 14-i uses x number of DS0j time slots as a signaling channel for transmitting control information between the base station 14-i and the BSC 12, where xxe2x89xa71. Each of the base stations 14-i can support enough user traffic to consume p number of traffic channels (for transmitting the user traffic between the base station 14-i and the BSC 12 and a voice switch, not shown), wherein a traffic channel comprises y number of DS0j time slots and yxe2x89xa71. Thus, the T1 facilities 16-i should at least have a bandwidth that includes q number of DS0j time slots, where q=yxc2x7p+x.
Generally, each of the T1 facilities 16-i has a bandwidth that includes more than q number of DS0j time slotsxe2x80x94that is, the user traffic supported by an individual base station (and the control information) does not consume the entire bandwidth available on a T1 facility (i.e., q less than n). Thus, the base station does not use some DS0j time slots resulting in inefficient utilization of the T1 facility.
To more efficiently utilize T1 facilities and reduce the cost of a wireless communication system, one or more T1 facilities may be shared by a plurality of base stations. FIG. 4 illustrates a wireless communication system 20 in which a daisy chain configuration is used to connect n number of base stations 22-i to a BSC 24 via a common T1 facility 26. Unlike the wireless communication system 10, the wireless communication system 20 has a total of 24 number of DS0j time slots (compared to 24xc2x7n number of DS0j time slots in the wireless communication system 10). Like the base stations 14-i, each of the base stations 22-i has its own signaling channel comprising x number of DS0j time slotsxe2x80x94that is, nxc2x7x number of DS0j time slots on the T1 facility 26 are used as signaling channels. The remaining DS0j time slots in the T1 facility 26, i.e., mxe2x88x92nxc2x7x, can be used as traffic channels. The number of remaining DS0j time slots should be at least equal to the total number of DS0j time slots needed by the base stations 22-i to support user traffic, i.e., 24xe2x88x92nxc2x7xxc2x7yxc2x7pxc2x7n. Recall that n represents the total number of base stations, x represents the number of DS0j time slots composing a signaling channel, y represents the number of DS0j time slots composing a traffic channel, and represents the number of traffic channels consumable by user traffic per base station.
In some cases, the total number of DS0j time slots needed by the base stations to support user traffic is greater than the number of remaining DS0j time slots. In these cases, more DS0j time slots (or traffic channels) are required to support the user traffic. For example, suppose the wireless communication system 20 includes five base stations (i.e., n=5), each signaling channel and traffic channel comprises one DS0j time slot (i.e., x=1 and y=1), and the user traffic supported by each of the base stations 22-i consumes four traffic channels (i.e., p=4). If each base station 2-i has its own signaling channel (for a total of five DS0j time slots), then there would only be nineteen DS0j time slots available for use as traffic channels by the five base stations. Since each base station supports enough user traffic to consume four traffic channels, twenty DS0j time slots are needed by the wireless communication system for traffic channels. Because the T1 facility has only nineteen DS0j time slots available for use as traffic channels, the needs of the wireless communication system 20 would exceed the capacity of the T1 facility.
One way to increase the number of DS0j time slots available for use as traffic channels is to use additional T1 facilities. Such a solution would undesirably increase the cost of the wireless communication system. Accordingly, there exists a need for increasing the number of DS0j time slots available for use as traffic channels in a T1 facility connecting two or more base stations to a base station controller.
The present invention provides an apparatus and a method for increasing the number of DS0j time slots available for use as traffic channels in a T1 or E1 facility connecting two or more base stations to a base station controller by sharing a signaling channel among the base stations. The present invention is based on the concept that a single base station does not utilize the entire bandwidth of the signaling channel for the transmittal of signaling messages, i.e., control information for managing resources at one or more base stations. Better utilization of the signaling channel bandwidth can be achieved if two or more base stations can share the same signaling channel which, in turn, results in making more DS0j time slots available fox use as traffic channels. To share a signaling channel, the wireless communication system should be configured such that each of the base stations can receive its signaling over a downlink the signaling channel, and transmit its signaling messages to the base station controller over an uplink signaling channel without interfering with signaling messages transmitted by other base stations over the uplink signaling channel.
In one embodiment, the base station controller transmits over the downlink signaling channel a downlink signaling message with an associated identifier which specifies the base station to which the signaling message is intended. Each of the base stations bridges or taps onto the transmission facility to receive the downlink signaling message and identifier. This permits downlink signaling messages and identifiers to pass transparently through the receiving base station and to other base stations in the wireless communication system. The receiving base station examines the identifier to determine whether the associated downlink signaling message is intended for that base station. If yes, the base station executes instructions contained within the downlink signaling message. Otherwise, the downlink signaling message is ignored.
In another embodiment of the present invention, each of the base stations transmits its uplink signaling message to the base station controller without interfering (e.g., overwriting) with uplink signaling messages transmitted by other base stations. In this embodiment, the base station receives uplink signaling messages (and associated identifiers) transmitted by other base stations over the uplink signaling channel to the bas station controller. The receiving base station stores the received uplink signaling message in memory associated with that base station. Also stored within the same (or different) memory may be an uplink signaling message (and identifier) generated by the receiving base station for transmission to the base station controller. Any signaling messages stored in the memory of the receiving base station is then transmitted to the base station controller on a xe2x80x9cfirst-in, first-outxe2x80x9d basis over the uplink signaling channel. Hence, signaling messages from all base stations in the wireless communication system are assured of being transmitted to the base station controller.